iconvconfig: Use the public feof_unlocked

Build of iconvconfig failed with CFLAGS=-Os since __feof_unlocked is
not a public symbol.  Replace with feof_unlocked (defined to
__feof_unlocked when IS_IN (libc)) to fix this.

Reported-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>